Use correct suffix for CRAN binaries

This commit fixes #18

If type = mac.binary, use .tgz
If type = win.binary, use .zip

Tests added in test-install-version

Passes when running R CMD check --as-cran

Note: other tests throw an error:

In untar(src, exdir = target, compressed = "gzip") :
    '/usr/bin/gnutar -xf '/var/folders/8q/pd5448hs7n92sy7d25lpnw600000gp/T//Rtmpx7aHMa/tempdisagg_0.13.tar.gz' -C '/var/folders/8q/pd5448hs7n92sy7d25lpnw600000gp/T//Rtmpx7aHMa/devtools337d2b2faa1c'' returned error code 127

 This error exists for me on the pristine copy from GitHub.

 Haven't debugged this error -- I'm assuming that R CMD check --as-cran passes on maintainer's system